The invention relates to a method for dewatering and/or degassing hydraulic fluids, a device for working the method, and use of the device.
Hydraulic fluids are used in hydraulic systems for both hydrostatic and hydrodynamic drives, found in particular in control, regulating, drive, and braking systems of vehicles, aircraft, and other devices. The following distinction is made between hydraulic fluids:
a) mineral-oil-based hydraulic fluids
b) glycols, glycol ethers, and glycerins
c) silicone oils and esters, and
d) slow-burning fluids.
The penetration of air into a hydraulic system results in functional problems and material wear by cavitation, especially in a hydrodynamic drive.
Some hydraulic fluids are hygroscopic, absorbing water, and when hydraulic fluid is heated considerably, as frequently occurs in hydraulic systems, it produces vapor. The latter in turn causes functional problems and material wear inside the hydraulic system.
To prevent these problems, the hydraulic fluid is either replaced from time to time, or degassed or dewatered periodically, or a device is used for continuous degassing or dewatering.
A degassing device of this kind is known for example from EP-B-0 168 656, with the line being divided into a passageway for fluid with a throttle valve and a passageway for separated gas with a degassing valve. However, this device has many moving parts and allows only a quasi-continuous removal of air, but no dewatering. The device described in D-B-29 37 957 for degassing hydraulic fluids is based on the principle of producing a vacuum in a reservoir and drawing off the gases contained in the hydraulic fluid, but this takes up space and is cost-intensive.
EP-A-0 155 115 describes a device which permits continuous removal of air from water without moving parts. This device has a porous PTFE membrane which however permits use only at low pressures with a maximum of 2.5 bars and thus is suited primarily for water and other fluids with similarly high surface tension. The device is therefore not suitable for degassing hydraulic fluids. The same applies to DE-E-A-37 08 950 which is likewise based on this principle, and to JP-A-4-156903 which relates to the use of a semipermeable membrane for degassing water.
GB-B-1341052 relates to a method and a device for recovering gaseous hydrocarbons when they escape from fuel tanks, using the permeability of a plastic tube to such hydrocarbons. In U.S. Pat. No. 4,897,091 as well, gaseous carbon dioxide is separated from methanol containing carbon dioxide by a membrane that is the reaction product of a polyamine with a polyisocyanate or polycarbonyl chloride.
DE-E-A-32 15 003 describes a dialysis device in which air is removed from an air-water mixture by a pressure differential between two chambers separated by a hydrophobic membrane.
A device for pressure equalization in closed chambers is known from DE-A-26 05 694, with a gas-permeable and liquid-impermeable seal consisting of very fine cotton cloth or similar material being impregnated with the polymer of a fluorinated or chlorinated hydrocarbon.
The goal of the invention therefore is to provide a method and a device for degassing and/or dewatering hydraulic fluids which permits permanent degassing and/or dewatering in a reliable fashion while occupying limited space and having a low cost.
This goal is achieved according to the invention by virtue of the fact that gases dissolved in hydraulic fluid and/or water contained in said fluid are separated from the hydraulic fluid by the principle of pervaporation at a membrane which is permeable to gas and/or water but is impermeable to hydraulic fluid, with a partial pressure drop being maintained in the vicinity for gases and/or water.
An improvement on the invention consists in the fact that a pressure is developed or an existing pressure is utilized to maintain a partial pressure differential within the hydraulic system.
According to the invention, provision is made to develop a vacuum outside the hydraulic system to maintain the partial pressure differential.
It is also advantageous that a pressure is developed inside the hydraulic system to maintain the partial pressure differential and a vacuum is produced outside the hydraulic system for the same purpose.
Provision is made according to the invention such that the temperature of the hydraulic fluid is elevated or an elevated temperature of the hydraulic fluid is utilized for dewatering the hydraulic fluid.
The framework of the invention also includes a device for working the method according to the invention characterized in that a membrane is provided inside the hydraulic system that is permeable to gas and/or water and is impermeable to hydraulic fluid.
It is advantageous that means are provided for producing a vacuum on the outside of the membrane facing away from the hydraulic system.
One design of the invention consists in the membrane being integrated into a tubular line segment of the hydraulic system.
Provision is also made according to the invention for the membrane to be cylindrical in shape.
It is advantageous for the membrane to be a solvent/diffusion membrane.
The framework of the invention likewise includes a membrane consisting of polypropylene (PP), polyethylene (PE), polyvinyltrimethylsiloxane (PVTMS), polytetrafluoroethylene (PTFE), polybutadiene (PB), polydimethylsiloxane (PDMS), or polychloroprene (PCP).
Finally, the use of the device for degassing and/or dewatering hydraulic fluid in control, regulating, drive, and brake hydraulic systems of motor vehicles, vessels, and aircraft is covered by the invention.
The advantages of the invention consist essentially in the fact that a reliable and cost-favorable, non-space-intensive possibility is created for degassing and/or dewatering hydraulic fluids which can be used for example when, for space reasons, the hydraulic lines cannot be laid so that they continuously rise but an arc must be taken into account at which the gas and/or steam bubbles can collect, which can result in a total failure of the hydraulic system, something which absolutely must be avoided.
An embodiment of the invention is described below with reference to the drawings.